1. Field of the Invention
The present invention relates to a liquid crystal display device and, specifically, to a liquid crystal display device having improved front surface luminance.
2. Description of the Related Art
A liquid crystal display device (hereinafter, also referred to as LCD) has been used in a broadening range of fields every year as an image display device which has low power consumption and saves spaces. A liquid crystal display device has a constitution in which a backlight (hereinafter, also referred to as BL), a backlight-side polarizing plate, a liquid crystal cell, a display-side polarizing plate, and the like are provided in this order.
Recently, for liquid crystal display devices, development for power saving, high definition, and color reproducibility improvement has been underway in order to improve LCD performance. At the moment, while there is a significant demand for power saving, high definition, and color reproducibility improvement particularly in small-sized display devices such as table PCs and smartphones, development of next-generation Hi-visions (4K2K, EBU ratio of 100% or higher) with current TV standards (FHD, 72% of National Television System Committee (NTSC) ratio≈100% of European Broadcasting Union (EBU) ratio) is also underway for large-sized display devices. Therefore, there is an intensifying demand for power saving, high definition, and color reproducibility improvement in liquid crystal display devices.
In accordance with power saving in the backlight, there are cases in which an optical sheet member is provided between the backlight and the backlight-side polarizing plate in order to increase light utilization efficiency. The optical sheet member is an optical element that, out of incidence light rays vibrating in random directions, transmits only light rays vibrating in a specific polarization direction and reflects light rays vibrating in other polarization directions. As a core member of a low-power LCD developed in response to an increasing number of mobile devices and a decrease in power consumption of home appliance, it is expected to increase luminance (the degree of brightness of a light source per unit area) by solving the low light utilization efficiency of LCDs.
As the above-described optical sheet member, a technique is known in which a specific optical sheet member (Dual Brightness Enhancement Film (DBEF) or the like) is provided between the backlight and the backlight-side polarizing plate so as to improve the light utilization efficiency of the BL by means of light recycling and thus the luminance of the BL is improved while saving power in the backlight (refer to JP3448626B). Similarly, JP1989-133003A (JP-H1-133003A) describes a polarizing plate obtained by laminating a λ/4 plate and a cholesteric liquid crystalline phase. It is possible to improve the light utilization efficiency of the BL by means of light recycling by broadening the bandwidths in a layer formed by fixing three or more layers of a cholesteric liquid crystalline phase having different pitches between the cholesteric liquid crystalline phases.
However, the above-described optical sheet member has a complicated member constitution, and thus, in order to distribute the optical sheet member in the market, it becomes essential to reduce the cost by decreasing the number of members by means of additional integration of functions of the members.
Meanwhile, from the viewpoint of high definition and color reproducibility improvement of a liquid crystal display device, a method for sharpening the light emission spectrum of the backlight is also known. For example, JP2012-169271A describes a method for increasing luminance and improving color reproducibility by realizing white light using a quantum dot (QD) which emits red light and green light as a fluorescent body between a blue LED and a light guide plate. In SID'12 DIGEST p. 895, a method of combining a light conversion sheet (QDEF, also referred to as quantum dot sheet) in which a quantum dot is used in order to improve the color reproducibility of the LCD is proposed.
In addition, in JP2012-22028A, a method is proposed in which a fluorescent body layer including a fluorescent body made up of quantum dots is disposed in a path for light radiated by a violet LED or a blue LED, thereby decreasing the energy loss in a color filter and increasing the energy efficiency of a liquid crystal display.
Meanwhile, in JP2012-502322A, a liquid crystal display device is proposed in which a blue light source, a cholesteric liquid crystal, a light conversion layer capable of altering the wavelength of light to a longer value, and a λ/4 plate are combined together, thereby providing clearly visible bright images under a bright ambient light condition with a low power consumption and improving long-term reliability.